A : pH of equimolar solution of `NH_4 CI and NH_4OH` does not change when small amount of HCI is added to it .
R : pOH of above solution is equal to `pK_b` of the buffer.

Options

A. If both Assertion & Reason are true and the reason in the corret explanation of the assertion, then mark
B. If both assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark
C. If Assertion is true statement but Reason is false then mark
D. If both Assertion and Reason are false statements , then mark

Answer

To solve the question, we need to analyze both the assertion (A) and the reason (R) provided.

### Step 1: Understanding the Assertion
The assertion states that the pH of an equimolar solution of NH₄Cl (ammonium chloride) and NH₄OH (ammonium hydroxide) does not change when a small amount of HCl (hydrochloric acid) is added to it.

- **Explanation**: NH₄OH is a weak base, and NH₄Cl is its salt. When mixed in equimolar amounts, they form a buffer solution. Buffers resist changes in pH upon the addition of small amounts of acids or bases. Therefore, the assertion is correct.

### Step 2: Understanding the Reason
The reason states that the pOH of the above solution is equal to the pK_b of the buffer.

- **Explanation**: For a buffer solution made from a weak base (NH₄OH) and its salt (NH₄Cl), the relationship between pOH and pK_b can be expressed using the Henderson-Hasselbalch equation:

\[ \text{pOH} = \text{pK}_b + \log\left(\frac{[\text{Salt}]}{[\text{Base}]}\right) \]

Since the solution is equimolar, the concentrations of NH₄Cl and NH₄OH are equal. Thus, the ratio \(\frac{[\text{Salt}]}{[\text{Base}]}\) equals 1, and the logarithm of 1 is 0. Therefore:

\[ \text{pOH} = \text{pK}_b + 0 = \text{pK}_b \]

This confirms that the reason is also correct.

### Step 3: Conclusion
Both the assertion and the reason are correct. However, the reason does not provide a complete explanation for the assertion. Therefore, the correct option is that both statements are true, but the reason is not the correct explanation for the assertion.

### Final Answer
- Assertion (A) is correct.
- Reason (R) is correct but does not explain (A).